This invention relates to a web transport arrangement, such as a tape transport system, and more particularly, to apparatus for controlling the tension in such a system in response to the detection of slip, thereby eliminating slip so that the web is transported at a substantially constant, desired speed.
Web transport systems are known wherein a web of material, such as a magnetic tape, a paper tape, a film, or the like, is transported between supply and take-up reels. In tape transport systems, for example, such as magnetic tape transports, information, such as audio information, is recorded on or reproduced from the tape as it is moved between the supply and take-up reels. Typically, audio signal recording/reproducing systems are provided with a capstan and pinch roller to engage the magnetic tape therebetween; and the rotation of the capstan serves to drive the tape in either the forward or reverse direction. As the tape is transported, audio signals are recorded thereon; or previously recorded signals are reproduced therefrom. The tape used in a conventional analog audio signal recording/reproducing system is transported at a relatively slow rate which, for example, is no greater than about seven inches per second.
Recently, digital audio signal recorders have been developed wherein an analog audio signal first is converted to digital form and then the digitized audio signal, which may be a pulse code modulated (PCM) signal, is recorded on the magnetic tape. Original audio sounds may be recorded and reproduced with higher fidelity when digital recording techniques are used than when analog recording techniques are adopted. However, the speed at which the magnetic tape is transported for digital recording/reproduction is far greater than the tape speed for analog recording/reproduction. For example, in a digital audio signal recording/reproducing system, the tape is transported at a speed on the order of about 76 cm./sec. At these and higher tape transport speeds, the conventional tape drive mechanism used in an analog recorder exhibits less than acceptable results. That is, at the relatively high tape transport speeds employed in a digital audio signal recorder/reproducer, the combination of a capstan and pinch roller to engage and drive the tape is not satisfactory.
It has been proposed in, for example, U.S. Pat. No. 4,232,257, to employ a capstan of relatively large diameter, without a pinch roller, to transport the magnetic tape at the necessarily high transport speeds for digital audio signal recording/reproducing techniques. The tape is maintained with appropriate tension against the capstan by selectively controlling the rotation of the supply and take-up reels with separate supply reel and take-up reel drive motors. Furthermore, guide rollers are provided to insure that the tape is wrapped about a sufficient peripheral portion of the capstan such that the friction between the tape and the capstan is adequate to drive the tape at the desired speed. Tape take-up tension between the capstan and the take-up reel is sensed; and the take-up reel drive motor is controlled as a function of this sensed tape take-up tension. Similarly, tape supply tension between the capstan and the supply reel is detected; and the supply reel drive motor is controlled as a function of this sensed tape supply tension. Consequently, the supply and take-up reels may be driven independently of each other, and at different speeds, so as to maintain proper tension in the tape. This, in turn, provides the appropriate friction between the tape and the capstan such that the tape may be driven by the capstan alone at the desired, high speed.
However, in tape transport systems of the type just described, the tape take-up and supply tensions must be maintained in proper balance. If, for example, the tape take-up tension is reduced relative to the tape supply tension, a slackening of the tape across the surface of the capstan may result. This slackening of the tape against the capstan may produce an air film therebetween, resulting in slippage between the tape and the capstan. Once formed, this air film is difficult, if not impossible, to remove without changing the speed at which the tape is driven. Hence, the formation of the air film not only introduces tape slip but, because of the inherent nature of the tape transport system, acts to maintain that tape slip so as to oppose the recovery of the normal tape transport speed.